Plotters are used in the computer-generated graphics, sign-making and screen-printing fields for a variety of purposes, including cutting graphic images or characters from sheets of vinyl or other polymeric materials, drawing lines, characters or other graphic images on sheets of paper or polymeric materials, pouncing sheets to form perforated patterns, and embossing sheets to form embossed patterns.
In a typical sprocket-driven plotter, a pair of drive sprockets engage corresponding feed holes formed in the marginal portions of a sheet for moving the sheet relative to a tool carriage supporting a cutting blade, pen or other type of tool. A drum having a diameter approximately equal to the diameter of the sprockets is rotatably mounted between the sprockets for supporting the laterally-extending section of the sheet engaged by the sprockets. The drum is typically mounted on a common sprocket shaft, and a servo motor or stepper motor is drivingly connected to the shaft to rotatably drive both the sprockets and drum and in turn move the sheet relative to the tool supported on the carriage.
One of the drawbacks of this type of prior art plotter is that the inertia generated upon rotating the drum and reflected back to the drive motor increases exponentially with increasing drum radii. Accordingly, for plotters having relatively large drum radii, the inertia reflected back to the drive motor creates a substantial load, and correspondingly, a substantial amount of torque and power may be required to accelerate the sprockets and drum at a desired level. Thus, it would be desirable to reduce the drum diameter in order to decrease the torque required to accelerate the plotter, and in turn decrease the size of the motor, power supply and overall weight of the plotter. However, it is necessary to maintain the drum diameter approximately equal to the sprocket diameter in order to conformably support with the drum the laterally-extending section of the sheet engaged by the sprockets. In addition, the sheets used with such plotters define a predetermined feed-hole pattern for receiving the sprocket pins. A reduction in the sprocket diameter would likely reduce the number of sprocket pins simultaneously engaging the sheets, and in turn negatively affect the ability of the sprockets to drive and steer the sheets.
Accordingly, it is an object of the present invention to provide a sprocket-driven plotter having a relatively low inertia reflected back to the drive motor in order to obtain similar or improved acceleration and plotter throughput without requiring an increase in the size of the motor and/or power supply in comparison to prior art drum-type plotters.